Water is a precious commodity, especially water that is sufficiently pure to be utilized for landscaping, agriculture or as potable water. As water becomes more scarce and water demand increases, potential sources for reclaiming of usable water warrant investigation. Large volumes of water are involved in evaporative cooling heat exchangers. One form of evaporative heat exchanger is an evaporative cooling tower (also called a wet air cooling tower). Evaporative cooling towers distribute water near the top of a tower having a hollow central core, typically sprayed into the tower. Air passes up through the tower (either forced by a fan or by natural convection), resulting in warm moist air being discharged from an upper end of the cooling tower. Falling water cooled by the transfer of moisture to the air is collected at a lower end of the cooling tower and is then usable as a heat exchange fluid to cool a working fluid within a power plant (or as a chilled water for air conditioning or for other uses). Such cooling towers are also utilized with air conditioning systems, often mounted upon the roof of a large building. For power plants, such cooling towers are typically freestanding.
One common attribute of such cooling towers is that they discharge relatively warm moist air therefrom. Exceptionally large amounts of water are lost to the atmosphere in the form of the humid air being discharged from the heat exchanger. Other forms of evaporative heat exchangers also include a discharge which includes moist air. This moist air is a source of moisture available for recovery.
As an example of the large amounts of water utilized to cool power plants, a rough estimate of the number of gallons of water used per day by the entire United States for all purposes is 400 billion gallons of water. A rough estimate of the number of gallons of water utilized in the cooling of power plants in the United States is 200 billion gallons per day. Thus, approximately half of all water usage in the Unites States is water usage within power plants. As water becomes more scarce, an increasing need exists to reclaim water used in power plants, and particularly water used in a condenser portion of thermal power plants, such as the large amounts of water utilized by evaporative cooling towers.
One known mechanism for altering the humidity of air is generally referred to as a dehumidifier. While dehumidifiers can come in a variety of different configurations, one common form of dehumidifier shares many attributes with an air conditioning system. In particular, relatively warm moist air has water extracted therefrom by first cooling the air to a lower temperature. Lower temperature air cannot hold as much water for a given volume as can warm air. Hence, by cooling the air, moisture precipitates out of the air as a liquid. The air has thus been effectively dehumidified. In many dehumidifiers, a second portion of the dehumidifier reheats the cooled and dehumidified air, such as back to an inlet temperature for the air. Such heating can occur by heat exchange with the warm moist air entering the dehumidifier to pre-cool this warm moist air and decrease the power requirements of the dehumidifier.
Regardless of the configuration of the dehumidifier, it serves the basic purpose of extracting moisture from the air. The effectiveness at which the dehumidifier removes water from the air is a function of how much moisture is in the air initially.